feat: accept null values

[askoura]

Description

This allows the form to send null values as acceptable answers, useful to erase an answer that was solved earlier. This was tested with a frontend build feat/ranking-mobile-reorder-controls... still in review until the backend is first merged

Type

• Bug fix
• New feature
• Refactor
• Documentation update

How to Test

1. Build the APK:

   ./gradlew assembleDebug

2. Install on emulator or device:

   ./gradlew installDebug

3. Test your changes:

   • Describe how to verify the fix works

Testing

• Unit tests pass (./gradlew test)
• Instrumentation tests pass (./gradlew connectedAndroidTest)
• Tested on emulator
• Tested on physical device (if hardware-specific)

Checklist

• Follows Kotlin coding conventions
• No new lint warnings
• No regressions in existing functionality
• Tested offline mode (if applicable)

Related Issue

Closes #

Additional Notes

[CLAassistant]

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